Field of the Invention
The present invention relates to a method for producing activated carbon, suitable in particular for use in double-layer condensers.
Activated carbon, also known as activated charcoal, has a very high porosity, and so is used particularly in for removing of unwanted dyes, flavoring agents and/or odorizing agents from gases and liquids, during wastewater treatment or air purification for example. In this context, the activated carbon may be used in the form of granulate, a powder or pellets depending on the application in question.
Besides this use, because of its high porosity activated carbon is also often used as an electrode material, for example in double-layer condensers, the use of which is becoming increasingly widespread because of their high energy density. Such double-layer condensers are configured with two electrodes, kept apart from one another by a separator and coated with electrolyte. In order to maximize the condenser's energy density, the electrodes used in the double-layer condensers must have the highest density possible as well as high porosity.
Activated carbon is generally produced by oxidative thermal activation at 600 to 1000° C., during which a portion of the carbon is transformed into carbon monoxide, which in turn creates more pores in the carbon, increasing the surface area of the substance further still. In this context, oxidative thermal activation usually takes place in the presence of a strong base, preferably potassium hydroxide or another alkali metal hydroxide, and this is why carbon that is activated in a method of this kind is also referred to as alkali-activated carbon or base-activated carbon.
One problem with using alkalis or bases such as potassium hydroxide to activate carbon consists in that a by-product of the oxidative thermal treatment of carbon with a base is a reduction product of the base, in the case of potassium hydroxide this by-product being metallic potassium, which is strongly corrosive and accordingly causes corrosion of the installation in which the activation is carried out.
In order to minimize this problem of corrosion, a method for producing activated carbon is suggested in published, European patent application EP 1 498 389 A1, corresponding to U.S. Pat. No. 7,709,415, in which a carbon material in powder form, for example pulverized pitch fibers, is mixed with potassium hydroxide particles. The mixture thus obtained is then granulated at a temperature of at least 80° C. and preferably under reduced pressure to form granulate particles having an average particle diameter not exceeding 50 mm, after which the granulate particles are dehydrated at a temperature of at least 200° C., preferably under reduced pressure, and the dehydrated granulate particles are then activated in a nitrogen stream at a temperature between 500 and 900° C. Then, the activated carbon particles may be combined with binding agents and filler materials as necessary and shaped into electrodes. This document also describes an alternative method, in which a mixture of carbon material in powder form and potassium hydroxide is subjected to thermal treatment up to a maximum of 300° C. and the mixture treated in this way is then compacted by compression molding into pellets having a diameter not exceeding 20 mm, and these pellets then undergo heat treatment at a temperature between 600 and 1,000° C. in a nitrogen stream. Then the pellets are washed with dilute hydrochloric acid and again with water to obtain activated carbon particles with a particle diameter of 20 μm, for example.
Although the methods described in published, European patent application EP 1 498 389 A1 result in less corrosion of the activation equipment than in corresponding methods in which activation is carried out using a potassium hydroxide fusion, corrosive potassium vapor is still produced in these methods and it is spread throughout the entire installation by the nitrogen stream, so that the potassium also accumulates in the cooler areas of the apparatus and causes corrosion there. Apart from this, this method is very complicated, mainly because of the need to create granulate, which preferably takes place at elevated temperature and reduced pressure.